Buggy code:
@ Controller @ RequestMapping ( MappingServiceController . URI ) public class MappingServiceController extends MolgenisPluginController { private static final Logger LOG = LoggerFactory . getLogger ( MappingServiceController . class ) ; public static final String ID = "mappingservice" ; public static final String URI = ( MolgenisPluginController . PLUGIN_URI_PREFIX ) + ( MappingServiceController . ID ) ; private static final String VIEW_MAPPING_PROJECTS = "view-mapping-projects" ; private static final String VIEW_ATTRIBUTE_MAPPING = "view-attribute-mapping" ; private static final String VIEW_SINGLE_MAPPING_PROJECT = "view-single-mapping-project" ; private static final String VIEW_CATEGORY_MAPPING_EDITOR = "view-advanced-mapping-editor" ; private static final String VIEW_ATTRIBUTE_MAPPING_FEEDBACK = "view-attribute-mapping-feedback" ; @ Autowired private MolgenisUserService molgenisUserService ; @ Autowired private MappingService mappingService ; @ Autowired private AlgorithmService algorithmService ; @ Autowired private DataService dataService ; @ Autowired private OntologyTagService ontologyTagService ; @ Autowired private SemanticSearchService semanticSearchService ; private ExecutorService executors ; public MappingServiceController ( ) { } @ RequestMapping public String viewMappingProjects ( Model model ) { } @ RequestMapping ( value = "//addMappingProject" , method = RequestMethod . POST ) public String addMappingProject ( @ RequestParam ( "mapping-project-name" ) String name , @ RequestParam ( "target-entity" ) String targetEntity ) { } @ RequestMapping ( value = "//removeMappingProject" , method = RequestMethod . POST ) public String deleteMappingProject ( @ RequestParam ( required = true ) String mappingProjectId ) { } @ RequestMapping ( value = "//removeAttributeMapping" , method = RequestMethod . POST ) public String removeAttributeMapping ( @ RequestParam ( required = true ) String mappingProjectId , @ RequestParam ( required = true ) String target , @ RequestParam ( required = true ) String source , @ RequestParam ( required = true ) String attribute ) { } @ RequestMapping ( value = "//addEntityMapping" , method = RequestMethod . POST ) public String addEntityMapping ( @ RequestParam String mappingProjectId , String target , String source ) { } private void autoGenerateAlgorithms ( EntityMapping mapping , String target , EntityMetaData sourceEntityMetaData , EntityMetaData targetEntityMetaData , Iterable < AttributeMetaData > attributes , MappingProject project ) { } @ RequestMapping ( value = "//removeEntityMapping" , method = RequestMethod . POST ) public String removeEntityMapping ( @ RequestParam String mappingProjectId , String target , String source ) { } @ RequestMapping ( value = "//saveattributemapping" , method = RequestMethod . POST ) public String saveAttributeMapping ( @ RequestParam ( required = true ) String mappingProjectId , @ RequestParam ( required = true ) String target , @ RequestParam ( required = true ) String source , @ RequestParam ( required = true ) String targetAttribute , @ RequestParam ( required = true ) String algorithm ) { } @ RequestMapping ( "//mappingproject//{id}" ) public String viewMappingProject ( @ PathVariable ( "id" ) String identifier , @ RequestParam ( value = "target" , required = false ) String target , Model model ) { } @ RequestMapping ( "//createIntegratedEntity" ) public String createIntegratedEntity ( @ RequestParam String mappingProjectId , @ RequestParam String target , @ RequestParam String newEntityName , Model model ) { } @ RequestMapping ( "//attributeMapping" ) public String viewAttributeMapping ( @ RequestParam ( required = true ) String mappingProjectId , @ RequestParam ( required = true ) String target , @ RequestParam ( required = true ) String source , @ RequestParam ( required = true ) String targetAttribute , @ RequestParam ( required = true ) boolean showSuggestedAttributes , Model model ) { } @ RequestMapping ( value = "//attributemappingfeedback" , method = RequestMethod . POST ) public String attributeMappingFeedback ( @ RequestParam ( required = true ) String mappingProjectId , @ RequestParam ( required = true ) String target , @ RequestParam ( required = true ) String source , @ RequestParam ( required = true ) String targetAttribute , @ RequestParam ( required = true ) String algorithm , Model model ) { } <START_BUG> @ RequestMapping ( value = "//advancedmappingeditor" , method = RequestMethod . GET ) <END_BUG> public String advancedMappingEditor ( @ RequestParam ( required = true ) String mappingProjectId , @ RequestParam ( required = true ) String target , @ RequestParam ( required = true ) String source , @ RequestParam ( required = true ) String targetAttribute , @ RequestParam ( required = true ) String sourceAttribute , @ RequestParam String algorithm , Model model ) { MappingProject project = mappingService . getMappingProject ( mappingProjectId ) ; MappingTarget mappingTarget = project . getMappingTarget ( target ) ; EntityMapping entityMapping = mappingTarget . getMappingForSource ( source ) ; AttributeMapping attributeMapping = entityMapping . getAttributeMapping ( targetAttribute ) ; model . addAttribute ( "mappingProject" , project ) ; model . addAttribute ( "entityMapping" , entityMapping ) ; model . addAttribute ( "attributeMapping" , attributeMapping ) ; FieldType sourceAttributeDataType = dataService . getEntityMetaData ( source ) . getAttribute ( sourceAttribute ) . getDataType ( ) ; Iterable < Entity > targetAttributeEntities = dataService . findAll ( dataService . getEntityMetaData ( target ) . getAttribute ( targetAttribute ) . getRefEntity ( ) . getName ( ) ) ; model . addAttribute ( "targetAttributeEntities" , targetAttributeEntities ) ; String targetAttributeIdAttribute = dataService . getEntityMetaData ( target ) . getAttribute ( targetAttribute ) . getRefEntity ( ) . getIdAttribute ( ) . getName ( ) ; model . addAttribute ( "targetAttributeIdAttribute" , targetAttributeIdAttribute ) ; String targetAttributeLabelAttribute = dataService . getEntityMetaData ( target ) . getAttribute ( targetAttribute ) . getRefEntity ( ) . getLabelAttribute ( ) . getName ( ) ; model . addAttribute ( "targetAttributeLabelAttribute" , targetAttributeLabelAttribute ) ; Iterable < Entity > sourceAttributeEntities = null ; String sourceAttributeIdAttribute = null ; String sourceAttributeLabelAttribute = null ; if ( ( sourceAttributeDataType instanceof XrefField ) || ( sourceAttributeDataType instanceof MrefField ) ) { sourceAttributeEntities = dataService . findAll ( dataService . getEntityMetaData ( source ) . getAttribute ( sourceAttribute ) . getRefEntity ( ) . getName ( ) ) ; sourceAttributeIdAttribute = dataService . getEntityMetaData ( source ) . getAttribute ( sourceAttribute ) . getRefEntity ( ) . getIdAttribute ( ) . getName ( ) ;

Fix:
@ RequestMapping ( value = "//advancedmappingeditor" , method = RequestMethod . POST )